This module provides access to Transport Layer Security (often known as “Secure
Sockets Layer”) encryption and peer authentication facilities for network
sockets, both client-side and server-side. This module uses the OpenSSL
library. It is available on all modern Unix systems, Windows, Mac OS X, and
probably additional platforms, as long as OpenSSL is installed on that platform.

Note

Some behavior may be platform dependent, since calls are made to the
operating system socket APIs. The installed version of OpenSSL may also
cause variations in behavior.

Warning

Don’t use this module without reading the Security considerations. Doing so
may lead to a false sense of security, as the default settings of the
ssl module are not necessarily appropriate for your application.

This section documents the objects and functions in the ssl module; for more
general information about TLS, SSL, and certificates, the reader is referred to
the documents in the “See Also” section at the bottom.

This module provides a class, ssl.SSLSocket, which is derived from the
socket.socket type, and provides a socket-like wrapper that also
encrypts and decrypts the data going over the socket with SSL. It supports
additional methods such as getpeercert(), which retrieves the
certificate of the other side of the connection, and cipher(),which
retrieves the cipher being used for the secure connection.

For more sophisticated applications, the ssl.SSLContext class
helps manage settings and certificates, which can then be inherited
by SSL sockets created through the SSLContext.wrap_socket() method.

Raised to signal an error from the underlying SSL implementation
(currently provided by the OpenSSL library). This signifies some
problem in the higher-level encryption and authentication layer that’s
superimposed on the underlying network connection. This error
is a subtype of OSError. The error code and message of
SSLError instances are provided by the OpenSSL library.

Takes an instance sock of socket.socket, and returns an instance
of ssl.SSLSocket, a subtype of socket.socket, which wraps
the underlying socket in an SSL context. sock must be a
SOCK_STREAM socket; other socket types are unsupported.

For client-side sockets, the context construction is lazy; if the
underlying socket isn’t connected yet, the context construction will be
performed after connect() is called on the socket. For
server-side sockets, if the socket has no remote peer, it is assumed
to be a listening socket, and the server-side SSL wrapping is
automatically performed on client connections accepted via the
accept() method. wrap_socket() may raise SSLError.

The keyfile and certfile parameters specify optional files which
contain a certificate to be used to identify the local side of the
connection. See the discussion of Certificates for more
information on how the certificate is stored in the certfile.

The parameter server_side is a boolean which identifies whether
server-side or client-side behavior is desired from this socket.

The parameter cert_reqs specifies whether a certificate is required from
the other side of the connection, and whether it will be validated if
provided. It must be one of the three values CERT_NONE
(certificates ignored), CERT_OPTIONAL (not required, but validated
if provided), or CERT_REQUIRED (required and validated). If the
value of this parameter is not CERT_NONE, then the ca_certs
parameter must point to a file of CA certificates.

The ca_certs file contains a set of concatenated “certification
authority” certificates, which are used to validate certificates passed from
the other end of the connection. See the discussion of
Certificates for more information about how to arrange the
certificates in this file.

The parameter ssl_version specifies which version of the SSL protocol to
use. Typically, the server chooses a particular protocol version, and the
client must adapt to the server’s choice. Most of the versions are not
interoperable with the other versions. If not specified, the default is
PROTOCOL_SSLv23; it provides the most compatibility with other
versions.

Here’s a table showing which versions in a client (down the side) can connect
to which versions in a server (along the top):

client / server

SSLv2

SSLv3

SSLv23

TLSv1

SSLv2

yes

no

yes

no

SSLv3

no

yes

yes

no

SSLv23

yes

no

yes

no

TLSv1

no

no

yes

yes

Note

Which connections succeed will vary depending on the version of
OpenSSL. For instance, in some older versions of OpenSSL (such
as 0.9.7l on OS X 10.4), an SSLv2 client could not connect to an
SSLv23 server. Another example: beginning with OpenSSL 1.0.0,
an SSLv23 client will not actually attempt SSLv2 connections
unless you explicitly enable SSLv2 ciphers; for example, you
might specify "ALL" or "SSLv2" as the ciphers parameter
to enable them.

The parameter do_handshake_on_connect specifies whether to do the SSL
handshake automatically after doing a socket.connect(), or whether the
application program will call it explicitly, by invoking the
SSLSocket.do_handshake() method. Calling
SSLSocket.do_handshake() explicitly gives the program control over the
blocking behavior of the socket I/O involved in the handshake.

The parameter suppress_ragged_eofs specifies how the
SSLSocket.recv() method should signal unexpected EOF from the other end
of the connection. If specified as True (the default), it returns a
normal EOF (an empty bytes object) in response to unexpected EOF errors
raised from the underlying socket; if False, it will raise the
exceptions back to the caller.

Returns num cryptographically strong pseudo-random bytes. Raises an
SSLError if the PRNG has not been seeded with enough data or if the
operation is not supported by the current RAND method. RAND_status()
can be used to check the status of the PRNG and RAND_add() can be used
to seed the PRNG.

Returns (bytes, is_cryptographic): bytes are num pseudo-random bytes,
is_cryptographic is True if the bytes generated are cryptographically
strong. Raises an SSLError if the operation is not supported by the
current RAND method.

Generated pseudo-random byte sequences will be unique if they are of
sufficient length, but are not necessarily unpredictable. They can be used
for non-cryptographic purposes and for certain purposes in cryptographic
protocols, but usually not for key generation etc.

Returns True if the SSL pseudo-random number generator has been seeded with
‘enough’ randomness, and False otherwise. You can use ssl.RAND_egd()
and ssl.RAND_add() to increase the randomness of the pseudo-random
number generator.

If you are running an entropy-gathering daemon (EGD) somewhere, and path
is the pathname of a socket connection open to it, this will read 256 bytes
of randomness from the socket, and add it to the SSL pseudo-random number
generator to increase the security of generated secret keys. This is
typically only necessary on systems without better sources of randomness.

Mixes the given bytes into the SSL pseudo-random number generator. The
parameter entropy (a float) is a lower bound on the entropy contained in
string (so you can always use 0.0). See RFC 1750 for more
information on sources of entropy.

Verify that cert (in decoded format as returned by
SSLSocket.getpeercert()) matches the given hostname. The rules
applied are those for checking the identity of HTTPS servers as outlined
in RFC 2818 and RFC 6125, except that IP addresses are not currently
supported. In addition to HTTPS, this function should be suitable for
checking the identity of servers in various SSL-based protocols such as
FTPS, IMAPS, POPS and others.

CertificateError is raised on failure. On success, the function
returns nothing:

Changed in version 3.3.3: The function now follows RFC 6125, section 6.4.3 and does neither
match multiple wildcards (e.g. *.*.com or *a*.example.org) nor
a wildcard inside an internationalized domain names (IDN) fragment.
IDN A-labels such as www*.xn--pthon-kva.org are still supported,
but x*.python.org no longer matches xn--tda.python.org.

Given the address addr of an SSL-protected server, as a (hostname,
port-number) pair, fetches the server’s certificate, and returns it as a
PEM-encoded string. If ssl_version is specified, uses that version of
the SSL protocol to attempt to connect to the server. If ca_certs is
specified, it should be a file containing a list of root certificates, the
same format as used for the same parameter in wrap_socket(). The call
will attempt to validate the server certificate against that set of root
certificates, and will fail if the validation attempt fails.

Possible value for SSLContext.verify_mode, or the cert_reqs
parameter to wrap_socket(). In this mode (the default), no
certificates will be required from the other side of the socket connection.
If a certificate is received from the other end, no attempt to validate it
is made.

Possible value for SSLContext.verify_mode, or the cert_reqs
parameter to wrap_socket(). In this mode no certificates will be
required from the other side of the socket connection; but if they
are provided, validation will be attempted and an SSLError
will be raised on failure.

Possible value for SSLContext.verify_mode, or the cert_reqs
parameter to wrap_socket(). In this mode, certificates are
required from the other side of the socket connection; an SSLError
will be raised if no certificate is provided, or if its validation fails.

Selects SSL version 2 or 3 as the channel encryption protocol. This is a
setting to use with servers for maximum compatibility with the other end of
an SSL connection, but it may cause the specific ciphers chosen for the
encryption to be of fairly low quality.

Whether the OpenSSL library has built-in support for the Server Name
Indication extension to the SSLv3 and TLSv1 protocols (as defined in
RFC 4366). When true, you can use the server_hostname argument to
SSLContext.wrap_socket().

However, since the SSL (and TLS) protocol has its own framing atop
of TCP, the SSL sockets abstraction can, in certain respects, diverge from
the specification of normal, OS-level sockets. See especially the
notes on non-blocking sockets.

SSL sockets also have the following additional methods and attributes:

If there is no certificate for the peer on the other end of the connection,
returns None.

If the binary_form parameter is False, and a certificate was
received from the peer, this method returns a dict instance. If the
certificate was not validated, the dict is empty. If the certificate was
validated, it returns a dict with several keys, amongst them subject
(the principal for which the certificate was issued) and issuer
(the principal issuing the certificate). If a certificate contains an
instance of the Subject Alternative Name extension (see RFC 3280),
there will also be a subjectAltName key in the dictionary.

The subject and issuer fields are tuples containing the sequence
of relative distinguished names (RDNs) given in the certificate’s data
structure for the respective fields, and each RDN is a sequence of
name-value pairs. Here is a real-world example:

To validate a certificate for a particular service, you can use the
match_hostname() function.

If the binary_form parameter is True, and a certificate was
provided, this method returns the DER-encoded form of the entire certificate
as a sequence of bytes, or None if the peer did not provide a
certificate. Whether the peer provides a certificate depends on the SSL
socket’s role:

for a client SSL socket, the server will always provide a certificate,
regardless of whether validation was required;

Returns a three-value tuple containing the name of the cipher being used, the
version of the SSL protocol that defines its use, and the number of secret
bits being used. If no connection has been established, returns None.

Get channel binding data for current connection, as a bytes object. Returns
None if not connected or the handshake has not been completed.

The cb_type parameter allow selection of the desired channel binding
type. Valid channel binding types are listed in the
CHANNEL_BINDING_TYPES list. Currently only the ‘tls-unique’ channel
binding, defined by RFC 5929, is supported. ValueError will be
raised if an unsupported channel binding type is requested.

Returns the protocol that was selected during the TLS/SSL handshake. If
SSLContext.set_npn_protocols() was not called, or if the other party
does not support NPN, or if the handshake has not yet happened, this will
return None.

Performs the SSL shutdown handshake, which removes the TLS layer from the
underlying socket, and returns the underlying socket object. This can be
used to go from encrypted operation over a connection to unencrypted. The
returned socket should always be used for further communication with the
other side of the connection, rather than the original socket.

An SSL context holds various data longer-lived than single SSL connections,
such as SSL configuration options, certificate(s) and private key(s).
It also manages a cache of SSL sessions for server-side sockets, in order
to speed up repeated connections from the same clients.

Load a private key and the corresponding certificate. The certfile
string must be the path to a single file in PEM format containing the
certificate as well as any number of CA certificates needed to establish
the certificate’s authenticity. The keyfile string, if present, must
point to a file containing the private key in. Otherwise the private
key will be taken from certfile as well. See the discussion of
Certificates for more information on how the certificate
is stored in the certfile.

The password argument may be a function to call to get the password for
decrypting the private key. It will only be called if the private key is
encrypted and a password is necessary. It will be called with no arguments,
and it should return a string, bytes, or bytearray. If the return value is
a string it will be encoded as UTF-8 before using it to decrypt the key.
Alternatively a string, bytes, or bytearray value may be supplied directly
as the password argument. It will be ignored if the private key is not
encrypted and no password is needed.

If the password argument is not specified and a password is required,
OpenSSL’s built-in password prompting mechanism will be used to
interactively prompt the user for a password.

An SSLError is raised if the private key doesn’t
match with the certificate.

Load a set of “certification authority” (CA) certificates used to validate
other peers’ certificates when verify_mode is other than
CERT_NONE. At least one of cafile or capath must be specified.

The cafile string, if present, is the path to a file of concatenated
CA certificates in PEM format. See the discussion of
Certificates for more information about how to arrange the
certificates in this file.

The capath string, if present, is
the path to a directory containing several CA certificates in PEM format,
following an OpenSSL specific layout.

Load a set of default “certification authority” (CA) certificates from
a filesystem path defined when building the OpenSSL library. Unfortunately,
there’s no easy way to know whether this method succeeds: no error is
returned if no certificates are to be found. When the OpenSSL library is
provided as part of the operating system, though, it is likely to be
configured properly.

Set the available ciphers for sockets created with this context.
It should be a string in the OpenSSL cipher list format.
If no cipher can be selected (because compile-time options or other
configuration forbids use of all the specified ciphers), an
SSLError will be raised.

Note

when connected, the SSLSocket.cipher() method of SSL sockets will
give the currently selected cipher.

Specify which protocols the socket should advertise during the SSL/TLS
handshake. It should be a list of strings, like ['http/1.1','spdy/2'],
ordered by preference. The selection of a protocol will happen during the
handshake, and will play out according to the NPN draft specification. After a
successful handshake, the SSLSocket.selected_npn_protocol() method will
return the agreed-upon protocol.

Load the key generation parameters for Diffie-Helman (DH) key exchange.
Using DH key exchange improves forward secrecy at the expense of
computational resources (both on the server and on the client).
The dhfile parameter should be the path to a file containing DH
parameters in PEM format.

This setting doesn’t apply to client sockets. You can also use the
OP_SINGLE_DH_USE option to further improve security.

Set the curve name for Elliptic Curve-based Diffie-Hellman (ECDH) key
exchange. ECDH is significantly faster than regular DH while arguably
as secure. The curve_name parameter should be a string describing
a well-known elliptic curve, for example prime256v1 for a widely
supported curve.

This setting doesn’t apply to client sockets. You can also use the
OP_SINGLE_ECDH_USE option to further improve security.

Wrap an existing Python socket sock and return an SSLSocket
object. sock must be a SOCK_STREAM socket; other socket
types are unsupported.

The returned SSL socket is tied to the context, its settings and
certificates. The parameters server_side, do_handshake_on_connect
and suppress_ragged_eofs have the same meaning as in the top-level
wrap_socket() function.

On client connections, the optional parameter server_hostname specifies
the hostname of the service which we are connecting to. This allows a
single server to host multiple SSL-based services with distinct certificates,
quite similarly to HTTP virtual hosts. Specifying server_hostname
will raise a ValueError if the OpenSSL library doesn’t have support
for it (that is, if HAS_SNI is False). Specifying
server_hostname will also raise a ValueError if server_side
is true.

Get statistics about the SSL sessions created or managed by this context.
A dictionary is returned which maps the names of each piece of information to their
numeric values. For example, here is the total number of hits and misses
in the session cache since the context was created:

Certificates in general are part of a public-key / private-key system. In this
system, each principal, (which may be a machine, or a person, or an
organization) is assigned a unique two-part encryption key. One part of the key
is public, and is called the public key; the other part is kept secret, and is
called the private key. The two parts are related, in that if you encrypt a
message with one of the parts, you can decrypt it with the other part, and
only with the other part.

A certificate contains information about two principals. It contains the name
of a subject, and the subject’s public key. It also contains a statement by a
second principal, the issuer, that the subject is who he claims to be, and
that this is indeed the subject’s public key. The issuer’s statement is signed
with the issuer’s private key, which only the issuer knows. However, anyone can
verify the issuer’s statement by finding the issuer’s public key, decrypting the
statement with it, and comparing it to the other information in the certificate.
The certificate also contains information about the time period over which it is
valid. This is expressed as two fields, called “notBefore” and “notAfter”.

In the Python use of certificates, a client or server can use a certificate to
prove who they are. The other side of a network connection can also be required
to produce a certificate, and that certificate can be validated to the
satisfaction of the client or server that requires such validation. The
connection attempt can be set to raise an exception if the validation fails.
Validation is done automatically, by the underlying OpenSSL framework; the
application need not concern itself with its mechanics. But the application
does usually need to provide sets of certificates to allow this process to take
place.

Python uses files to contain certificates. They should be formatted as “PEM”
(see RFC 1422), which is a base-64 encoded form wrapped with a header line
and a footer line:

The Python files which contain certificates can contain a sequence of
certificates, sometimes called a certificate chain. This chain should start
with the specific certificate for the principal who “is” the client or server,
and then the certificate for the issuer of that certificate, and then the
certificate for the issuer of that certificate, and so on up the chain till
you get to a certificate which is self-signed, that is, a certificate which
has the same subject and issuer, sometimes called a root certificate. The
certificates should just be concatenated together in the certificate file. For
example, suppose we had a three certificate chain, from our server certificate
to the certificate of the certification authority that signed our server
certificate, to the root certificate of the agency which issued the
certification authority’s certificate:

If you are going to require validation of the other side of the connection’s
certificate, you need to provide a “CA certs” file, filled with the certificate
chains for each issuer you are willing to trust. Again, this file just contains
these chains concatenated together. For validation, Python will use the first
chain it finds in the file which matches. Some “standard” root certificates are
available from various certification authorities: CACert.org, Thawte, Verisign, Positive SSL
(used by python.org), Equifax and GeoTrust.

In general, if you are using SSL3 or TLS1, you don’t need to put the full chain
in your “CA certs” file; you only need the root certificates, and the remote
peer is supposed to furnish the other certificates necessary to chain from its
certificate to a root certificate. See RFC 4158 for more discussion of the
way in which certification chains can be built.

Often the private key is stored in the same file as the certificate; in this
case, only the certfile parameter to SSLContext.load_cert_chain()
and wrap_socket() needs to be passed. If the private key is stored
with the certificate, it should come before the first certificate in
the certificate chain:

If you are going to create a server that provides SSL-encrypted connection
services, you will need to acquire a certificate for that service. There are
many ways of acquiring appropriate certificates, such as buying one from a
certification authority. Another common practice is to generate a self-signed
certificate. The simplest way to do this is with the OpenSSL package, using
something like the following:

This example connects to an SSL server and prints the server’s certificate:

importsocket,ssl,pprints=socket.socket(socket.AF_INET,socket.SOCK_STREAM)# require a certificate from the serverssl_sock=ssl.wrap_socket(s,ca_certs="/etc/ca_certs_file",cert_reqs=ssl.CERT_REQUIRED)ssl_sock.connect(('www.verisign.com',443))pprint.pprint(ssl_sock.getpeercert())# note that closing the SSLSocket will also close the underlying socketssl_sock.close()

As of January 6, 2012, the certificate printed by this program looks like
this:

(it is assumed your operating system places a bundle of all CA certificates
in /etc/ssl/certs/ca-bundle.crt; if not, you’ll get an error and have
to adjust the location)

When you use the context to connect to a server, CERT_REQUIRED
validates the server certificate: it ensures that the server certificate
was signed with one of the CA certificates, and checks the signature for
correctness:

For server operation, typically you’ll need to have a server certificate, and
private key, each in a file. You’ll first create a context holding the key
and the certificate, so that clients can check your authenticity. Then
you’ll open a socket, bind it to a port, call listen() on it, and start
waiting for clients to connect:

When a client connects, you’ll call accept() on the socket to get the
new socket from the other end, and use the context’s SSLContext.wrap_socket()
method to create a server-side SSL socket for the connection:

And go back to listening for new client connections (of course, a real server
would probably handle each client connection in a separate thread, or put
the sockets in non-blocking mode and use an event loop).

When working with non-blocking sockets, there are several things you need
to be aware of:

Calling select() tells you that the OS-level socket can be
read from (or written to), but it does not imply that there is sufficient
data at the upper SSL layer. For example, only part of an SSL frame might
have arrived. Therefore, you must be ready to handle SSLSocket.recv()
and SSLSocket.send() failures, and retry after another call to
select().

(of course, similar provisions apply when using other primitives such as
poll())

The SSL handshake itself will be non-blocking: the
SSLSocket.do_handshake() method has to be retried until it returns
successfully. Here is a synopsis using select() to wait for
the socket’s readiness:

CERT_NONE is the default. Since it does not authenticate the other
peer, it can be insecure, especially in client mode where most of time you
would like to ensure the authenticity of the server you’re talking to.
Therefore, when in client mode, it is highly recommended to use
CERT_REQUIRED. However, it is in itself not sufficient; you also
have to check that the server certificate, which can be obtained by calling
SSLSocket.getpeercert(), matches the desired service. For many
protocols and applications, the service can be identified by the hostname;
in this case, the match_hostname() function can be used.

In server mode, if you want to authenticate your clients using the SSL layer
(rather than using a higher-level authentication mechanism), you’ll also have
to specify CERT_REQUIRED and similarly check the client certificate.

Note

In client mode, CERT_OPTIONAL and CERT_REQUIRED are
equivalent unless anonymous ciphers are enabled (they are disabled
by default).

SSL version 2 is considered insecure and is therefore dangerous to use. If
you want maximum compatibility between clients and servers, it is recommended
to use PROTOCOL_SSLv23 as the protocol version and then disable
SSLv2 explicitly using the SSLContext.options attribute:

If you have advanced security requirements, fine-tuning of the ciphers
enabled when negotiating a SSL session is possible through the
SSLContext.set_ciphers() method. Starting from Python 3.2.3, the
ssl module disables certain weak ciphers by default, but you may want
to further restrict the cipher choice. For example:

The !aNULL:!eNULL part of the cipher spec is necessary to disable ciphers
which don’t provide both encryption and authentication. Be sure to read
OpenSSL’s documentation about the cipher list
format.
If you want to check which ciphers are enabled by a given cipher list,
use the opensslciphers command on your system.

If using this module as part of a multi-processed application (using,
for example the multiprocessing or concurrent.futures modules),
be aware that OpenSSL’s internal random number generator does not properly
handle forked processes. Applications must change the PRNG state of the
parent process if they use any SSL feature with os.fork(). Any
successful call of RAND_add(), RAND_bytes() or
RAND_pseudo_bytes() is sufficient.